In general, an alkaline primary battery comprises a separator between a cathode active material and an anode active material for separating these materials. Such a separator need to satisfy various characteristics such that:                (1) preventing internal short circuit between the cathode and anode materials,        (2) enhancing absorption of electrolytes for sufficient electrogenic reaction as well as having a good ion conductivity to cause low electric resistance,        (3) requiring only small space in batteries, resulting in having the amount of cathode and anode active materials increased inside the battery (i.e., prolonging the life time of the battery), and so on.        
For example, Japanese Laid-open Patent Publication No. 6-163024 discloses a separator for batteries in order to satisfy the above requirements. The separator comprises a polyvinyl alcohol fiber and a cellulose fiber in combination, in which the polyvinyl alcohol fiber is excellent in chemical resistance, hydrophilic property, and mechanical property, and the cellulose fiber is used for enhancing absorption of electrolyte. The separator disclosed therein can, however, exhibit some disadvantageous characteristics that the separator produces a short circuit failure due to needle-like zinc oxide deposition, (i.e., dendrite formation) by an action of aluminum added to zinc which constitutes an anode. The aluminum is added for reducing the hydrogen generation in the electrolyte, but it also causes the dendrite. As a result, the battery using the separator induces an unusual voltage reduction, and the lifetime of the battery would be reduced.
Conventional alkaline-manganese batteries include a cathode mix of manganese dioxide and graphite and a separator comprising synthetic fibers and cellulose fibers. However, under high temperature storage, the cellulose fibers existing on the contact surface of the cathode mix are degraded by oxidation, and the decline of cathode capacity due to the cellulose fiber degradation would cause deterioration of the battery performance.
In addition, it would be desirable to have an alkaline battery that would provide improved battery performance characteristics, as various digital equipments have been rapidly widespread and increasingly sophisticated in recent years. Since the digital equipments utilizing these batteries need greater and greater electric power supplies, it would be desirable to have a battery excellent in discharge performance under high loading. Responding to such a request, an alkaline battery which is improved in discharge performance under high loading has been put to a practical use. Such a battery using a cathode active material comprising manganese dioxide in combination with nickel oxyhydroxide, however, exhibit some disadvantageous characteristics that the nickel oxyhydroxide, showing higher oxidizability than manganese dioxide, attacks cellulose fibers in the separator which exist on the contact surface of the cathode mix. As a result, the cellulose fibers are extremely rapidly degraded by oxidation and the performance of the battery is decreased under high loading.
In order to prevent internal short circuit between cathode and anode active materials, Japanese Laid-open Patent Publication No. 10-92411 suggests a battery separator having a bilayer structure in which a dense layer having a high fiber density and a rough layer (for storage a liquid material) having a low fiber density are used in combination. The separator is made from a refined polyvinyl alcohol fiber and a refined cellulose fiber as an alkali-resistant fiber.
However, in the above-mentioned separator, it is difficult to achieve a good balance between the liquid storage and the prevention of internal short circuit. The higher proportion of the dense layer in the separator would prevent short circuit caused by dendrite formation, but the separator was deteriorated in liquid storage ability. As a result, exhaustion (i.e., dry-out) of the liquid material can cause internal short circuit and adversely affect the battery life. Further, if the dense layer utilizes highly refined cellulose fibers, the separator not only reduces its strength, but also easily buckles by vibration or fallen impact on transferring or carrying the battery, resulting in internal short circuit in the battery. Moreover, the cellulose fibers on the contact surface with manganese dioxide as a cathode active material are degraded by oxidation, thereby the battery life becomes shorten. The tendency of oxidation degradation of the cellulose fibers is severer in high performance batteries which utilize a cathode mix comprising nickel oxyhydroxide which is excellent in discharge performance under high loading, and the degradation of the cellulose fibers can cause declining of the discharge performance under high loading of the battery.
On the contrary, in order to prevent internal short circuit of batteries, there has been adopted a separator comprising a paper substrate containing an alkali-resistant fiber and a cellulose fiber, in combination with a cellophane film. However, the separator is deteriorated in liquid storage capacity, and needs a number of paper substrates to ensure sufficient liquid storage amount. Therefore, the volume of the separator inside the battery has increased, and thereby this increase in volume of the separator significantly restricts the amounts of cathode and anode active materials in the battery. Further, the distance between the cathode and the anode would be longer by the separator which uses the cellophane film and increases the volume of the paper substrates, and the battery can invite rise in internal resistance. Accordingly, it has been found difficult to obtain a battery with high discharge performance.
Japanese Laid-open Patent Publications No. 57-105957, No. 57-105958 and No. 2-78150, for example, disclose a separator for batteries, which comprises a cross-linked high water-absorption polymer which is allowed to be applied or immersed in the rate of 0.5 to 10.0 g/m2. However, the separator disclosed therein cannot inhibit the dendrite formation, and has problems in that the separator is unable to sufficiently suppress short circuit due to dendrite formation and causes internal short circuit in the battery.
Further, Japanese Laid-open Patent Publications No. 2005-264420 and No. 2006-244804, for example, disclose a separator comprising a fiber having a nano-level diameter (hereinafter referred to as a nanofiber). The nanofibers in the separator disclosed therein are effective in prevention of the internal short circuit caused by the dendrite formation. However, absorption of electrolyte only depends on capillary action of minute void spaces in the nanofibers, and the nanofibers cannot absorb the electrolyte by themselves. As a result, it is difficult to prevent the separator from the electrolyte dry-out at the end of discharge.